This invention relates to an exhaust gas cleaning system for an internal combustion engine and, more particularly, to an exhaust gas cleaning system for reducing noxious components in the exhaust gases with use of a catalytic converter disposed in the engine exhaust system, to which secondary air is supplied. In the cleaning system, the secondary air is partly supplied to the engine intake system particularly in the overheating operation of the catalytic converter so as to produce a leaner air-fuel mixture to thereby reduce formation of hydrocarbons (HC) and carbon monoxide (CO).
In the prior art exhaust gas cleaning system equipped with a catalytic converter, upon reaching of a predetermined high temperature of the catalytic converter, the secondary air, which has been supplied in advance to the upstream side of the catalytic converter, is discharged to the atmosphere to stop reactions in the catalytic converter to thereby prevent the overheating thereof.
However, if such catalytic converter primarily for effecting oxidization is used in a reducing atmosphere without supply of the secondary air, it is liable to produce ammonia or other undesired products. Hence, it is desirable to place the exhaust gases in an oxidizing atmosphere with the supply of secondary air even during the overheating operaion of the catalytic converter.
On the other hand, in an engine having an exhaust gas recirculating system, condensation of moisture within the engine is probable, since the exhaust gases are always supplied to the intake system.